This invention relates generally to disc brake systems for motor vehicles.
The disc brake, the most common brake system in use in motor vehicles, operates by the friction of a pair of calipers pressing against the reverse sides of a rotating disc. A caliper is a device comprising a metal plate to which is mounted a frictional pad that presses against one side of a rotating disc when pressure is applied. The disc, which is a frictional ring positioned between the calipers and attached to a wheel of the vehicle, rotates with the wheel. One disc brake is positioned at each of the rear wheels or each of the front wheels, or, as is most common, at both pairs of the front and wheel wheels.
The disc brake system includes a brake housing that holds both a piston apparatus and the pair of calipers positioned around the disc. The piston apparatus is activated by a fluid brake system that is connected to a wheel brake fluid box in turn connected to the main brake fluid box.
Several problems exist in the brake disc systems now in use. One problem arises when the caliper pads are worn and must be replaced. The front wall of the piston extends slightly beyond the side wall of the housing for two reasons: First, the piston must be in constant contact with a movable caliper so as to be in position to react instantly upon activation of the brake by the driver without any slack to overcome. Second, as the caliper pads become worn, the front wall of the piston extends even more from the housing so as to overcome the space created by the worn away portion of the pads. A third reason common is that most of disc brake systems are designed so that the piston presses hard enough against the calipers to cause the wheel disc to constantly drag to some degree even when the wheel brake is not being operated; this drag design creates a pronounced ready-to-operate mode of the disc brakes. Thus, for the various reasons set forth above, the piston must be moved back into the housing in order to make room to accommodate the calipers after the new brake pads have been mounted to the metal plates of the calipers. The correct method of preparing the system for the new brake pads is generally as follows: First,the mechanic removes the emergency brake securing nut from the actuator screw at the rear of the housing; second, he rotates the actuator screw so that it pulls back the inner piston to which it is screwed; third, he removes the calipers from the wheel and replaces the brake pads; fourth, he physically presses the outer piston into the housing, a task made possible by the pulling back of the inner piston; fifth, he mounts the repaired calipers to the housing; fifth, he rotates the actuator screw in the opposite direction than in the earlier step so as to press the inner piston against the outer piston and so move the front wall of the outer piston against the calipers; and sixth, he remounts the emergency brake to the housing and replaces the emergency brake securing nut to the actuator screw. This process takes about 30 to 40 minutes for a skilled mechanic.
In this context, another problem often arises, namely, that created by an incompletely trained mechanic, who may understand the disc brake system to some extent, but who does not quite understand the relationship of the actuator screw and the emergency brake apparatus to forcing the piston back into the brake housing. It is unfortunately very common for the untrained mechanic actually to drain the brake fluid from the brake housing and then to pull the entire piston from the housing, then later to force the piston back into the housing, with the result that the rubber boot seal around the outer piston at the end of the housing is almost destroyed, among other consequences of such a misguided procedure. Such a process can often take two hours.
Another problem that exists in disc brake systems in current use is that the actuating area between the inner and outer pistons is accessible to air, humidity, and dust particles that pass through the outer oil hole at the front wall of the outer piston, even though a dust seal is inserted into the hole. The inner piston is sealed from the outer piston at a position inwards relative the actuating area. The result is that when the inner piston is caused to rotate upon the actuator screw upon application of fluid brake pressure, an action meant to force the outer piston against the nearest caliper plate by pressure around the actuating area, resistance to rotation of the inner piston created by rusting or frictionally retarding particles in the actuating area is often encountered. Two results are possible; One, the disc brake of one wheel will respond faster than the other due to an imbalance created with the disc brake on the other wheel; or two, there will be a slack response in the brake system in general.
A third problem encountered in disc brake systems in use is that in general it can be said that they are "drag" systems with a complex biasing structure in the housing so that continuous drag is created by the piston being biased against the calipers. Drag systems cause both excessive wearing of the brake pads and also add to operating cost of the vehicle since somewhat more power is needed to move the vehicle.